In recent years, the development of digital technology enables electronic devices such as portable information devices and information home appliances to have high functionality. Along with the expansion of functionality of such electronic devices, rapid progress is being made in miniaturizing semiconductor devices to be used and increasing the speed thereof. In particular, the usage of large-capacity non-volatile memory represented by a flash memory is being expanded rapidly. Furthermore, as a new, next-generation non-volatile memory which substitutes for the above flash memory, research and development are conducted on a non-volatile storage device in which a variable resistance element is used (which is referred to as a variable resistance non-volatile storage device or simply a non-volatile storage device). Here, a variable resistance element is an element which has a feature of reversibly changing its resistance value according to an electrical signal, and can furthermore store therein information corresponding to this resistance value in a non-volatile manner.
A crosspoint non-volatile memory element has been proposed, as an example of a large-capacity non-volatile memory which includes such a variable resistance element. An element having a configuration in which a variable resistance film is used as a storage unit and a diode element is used as a switching element is disclosed (see Patent Literature (PTL) 1, e.g.).
Parts (a) and (b) of FIG. 23 illustrate a non-volatile storage device 80 which includes conventional variable resistance elements. Part (a) of FIG. 23 is a perspective view of a crosspoint memory cell array 200 which includes bit lines 210, word lines 220, and memory cells 280 formed at crosspoints thereof. Part (b) of FIG. 23 is a cross-sectional view of the memory cell 280, the bit line 210, and the word line 220 in the bit line direction.
A variable resistance layer 230 which stores information according to a change in the electric resistance due to electrical stress is interposed between an upper electrode 240 and a lower electrode 250, thereby forming a variable resistance element 260. A nonlinear element 270 for bidirectional current flow is formed on the variable resistance element 260, and the memory cell 280 is formed by a series circuit having the variable resistance element 260 and the nonlinear element 270, the nonlinear element 270 having two terminals and nonlinear current-voltage characteristics. The nonlinear element 270 is a two-terminal element, such as a diode, having nonlinear current-voltage characteristics which show that a current change with respect to a voltage change is not constant.
In addition, the bit line 210 serving as an upper line is electrically connected to the nonlinear element 270, and the word line 220 serving as a lower line is electrically connected to the lower electrode 250 of the variable resistance element 260. A current flows through the nonlinear element 270 bidirectionally when the memory cell 280 is rewritten, and thus for example, a varistor having nonlinear current-voltage characteristics which are bidirectionally symmetrical (such as ZnO or SrTiO3) is used therefor. The above configuration allows a current flow of 30 kA/cm2 or more, which is current density necessary for rewriting the variable resistance element 260, and achieves large current density, thereby obtaining a large-capacity non-volatile memory.